1. Field of the Invention
This invention generally relates to surface modification devices and techniques, and more particularly to a background matching camouflage system. Specifically, the invention relates to a passive optical and infrared camouflage system in which a body is heated or cooled to match background emmissivity and in which the body may be simultaneously altered optically to correspond with background colors.
2. Background of the Invention
The art of concealment by altering an object to blend with its physical surroundings and environment, i.e., camouflage, has been practiced for centuries. Initially, it was only necessary to conceal an object from the visible physical surroundings. However, as technology has developed, it has become necessary to conceal an object over multiple bands of the electromagnetic spectrum. Most notably, in addition to visible concealment, it has become necessary to conceal an object""s infrared radiation (IR) to prevent thermal detection systems and the like from identifying an object based on its heat signature. Thus, modeling of camouflage effectiveness should consider both the infrared and visible spectrums.
There are three capabilities that must be addressed in a camouflage system: passive surveillance capability, active surveillance capability and high energy weapon capability. Of these, passive surveillance systems utilize electro-optical systems operating in the visible wavelength and infrared wavelength bands. Visual surveillance systems operate in the 0.4 to 0.7 micrometer portion of the electromagnetic spectrum. These systems rely on the visual, that is, that which is recognizable by the human eye. In addition, optical augmentation systems, which range from hand-held binoculars to video display terminals with zoom-in capability, may be utilized to enhance visual detection. In any event, detection mechanisms employed in visual surveillance systems employ color and/or brightness contrast to xe2x80x9cidentifyxe2x80x9d targets.
Passive systems which operate in the infrared wavelength bands, the 0.8 to 14 micrometer portion of the electromagnetic spectrum, which include the solar band, the high temperature band and the low temperature band, operate by homing-in on the contrast between the target IR signature and the IR signature of the surrounding environment.
Turning first to visible camouflage, making targets hard to find in the visible light spectrum (wavelength from 400-700 millimicrons) is primarily concerned with the development of ever more effective camouflage patterns and with techniques for characterizing the effectiveness of the camouflage for particular terrain. The techniques in use today largely involve painting, coloring, and/or contour shaping to allow an object to better blend with the surrounding environment. Other than color adaption to the background, these techniques have involved obscuring the contours of an object by covering the object with camouflage material such as nets or leaf cut tarpaulins. Such covering camouflage is good for visual concealment because the outlines of covered objects are disguised and difficult to discern from the surrounding natural environment, provided that the color scheme is harmonized with the surrounding natural environment. Thus, there are manufactured special nets for woodlands, for deserts, and for snow, all of which have very different color schemes.
However, in the visible spectrum, successful camouflage may be limited by factors including the following:
a. Camouflage patterns painted on a conventional surface are unable to change and a fixed camouflage pattern is inappropriate for the variety of backgrounds encountered in nature or otherwise man made.
b. One observer sees a military target against a rocky background while another observer sees the target against a forested background, while a third observer sees the target against a red barn. The current state of the art does not allow the military target to be effectively camouflaged for all these observers simultaneously or in real time.
c. When either the object or the observer moves, the background against which the target is seen changes, reducing the effectiveness of the camouflage pattern.
d. Most camouflage paints, irrespective of their color in the visible spectral range, tend to have high emissivities in the infrared spectral regions, wherein such emmissivities are significantly higher than those of most naturally occurring backgrounds. Therefore, targets painted with such paints can be clearly detected by imaging devices operating in the infrared spectral ranges.
Even the combination of several techniques may not effectively camouflage an object from detection. For example, known camouflage covering material, such as nets, generally have a very open, apertured structure. The proportionate covering of such conventional materials is only about 50-65%. This has been found to be insufficient when surfaces with high emmissivities, such as camouflage paints, are being covered because the high emissivities are still detectable through the covering""s apertures. Likewise, such coverings would also be ineffective in masking warm objects against detection by thermal reconnaissance.
Turning now to camouflage in the IR spectrum, finding targets in the IR spectrum utilizes target size and apparent temperature differences between the target and the background (known as xcex94T), a summary measure that combines target background physical temperature difference and target background emmissivity difference. Some targets contain highly concentrated heat sources which produce very high localized temperatures. There are also targets that contain a large number of heat sources with distinctive shapes which form easily recognizable patterns. As the contrast sensitivity of solid state detectors improves, it becomes possible to discern, for example, the number of cylinders in a gasoline engine and other subtle distinctions such as a change in fabrication material or perhaps a particular type of seam.
More specifically, many targets have internal heat sources which create a temperature contrast with the natural background which further enhanced the detectability of such targets by means of infrared sensing devices. For example, a tank generates large amounts of heat in the engine compartment and exhaust pipe, as well as from electric generators and motors. When the guns are fired, their barrels become heat sinks. Friction while the tank is moving heats the rims of the drive and the idler wheels and their central bearing portions. The track also becomes heated by friction with the wheels. The bearing area between the turret and tank body can also become heated. Moreover, radiant energy from the sun may be absorbed by the steel shell of a tank during the daytime, and at nighttime such energy reradiates from the shell, providing a clear IR signature against a cool background such as trees or hills. In addition, as mentioned above, the emissivities of paints tend, on average, to be significantly higher than those of most naturally occurring backgrounds. Therefore, a tank painted with camouflage paints can be clearly detected by imaging devices operating in the infrared spectral ranges.
To mask xcex94T differences, some IR camouflage prior art techniques have involved the use of subsystems to alter the surface of the object, such as forcing heated or cooled air over an object to match the object""s temperature to that of the surrounding environment. Of course, these subsystems themselves often have extraordinary power requirements which generate their own IR signature. Another technique has been to deploy decoy IR sources in an environment to radiate IR signatures equal to that of any specific target. More commonly, however, IR camouflage prior art techniques involve complete covering or shielding of an object with a material cover, such as a tarpaulin, in order to hide an objects IR signature.
Much effort has been expended in the determination of materials to be used to comprise the typical IR camouflage shielding. Typically, shielding provided only by a camouflage material cover will result in heating of the object covered by the material, such that while the structure and contours of such an object cannot be observed visually, the higher temperature of any exposed surface will be vulnerable to detection by IR detection devices. To overcome this effect, double-layered cover structures are utilized, wherein the outer, exposed camouflage material is insulated from a covered source of heat by a layer of insulating material arranged under and spaced apart from the outer material. Of course, the exposed outer camouflage material may still be heated or cooled by external conditions, yielding an IR signature that differs from the surrounding environment.
Thus, the above-described IR camouflage techniques have had only limited success due to factors such as the following:
a. Camouflage material has different heat transfer characteristics from the background resulting in changing apparent temperature differences between the target and the background over a given time interval.
b. Camouflage net material is vented to prevent heat build up, but winds cause the material to move which results in a blinking IR signature that is a clear beacon for detection.
c. One observer seeing an object against a hot background (such as the ground) and a second observer seeing the same object against a cold background (such as the sky), allows for a situation where the current state of the art does not permit the object to simultaneously be made to appear hotter to the first observer and colder to the second observer, and
d. When either the object and/or the observer moves, the apparent temperature and spatial pattern of the background against which the surface is seen appears to change, thus clearly revealing the target.
While the prior art teaches the use of surface modification devices and techniques, none have established a basis for a specific apparatus and technique dedicated to the task of resolving the particular problem at hand, namely a camouflage system to prevent both visual and IR detection. The methods and apparatus of the prior art both in the visible and infrared spectral ranges suffer from the drawback that the effective emmissivity of the camouflage material in the infrared ranges cannot readily be closely adapted to that of the surroundings from which the target should be indistinguishable when viewed by infrared detection equipment. Moreover, the thermal xe2x80x9csignaturexe2x80x9d of such targets resulting from internal heat sources such as internal combustion engines, exhaust pipes, electric motors or generators, or transformers, cannot readily be disguised by known methods. What is needed in this instance is a passive, real-time control of: 1) the effective emmissivity (band average and spectral) in the thermal wavelength region, 2) apparent color in the visible wavelength region, and 3) camouflage patterns for both thermal and visible wavelength regions.
The object of the present invention therefore is to provide means and a method for structuring the camouflage surface in such a manner that there is both color adaptation in the visual range and effective emmissivity in the infrared range which can simulate that of virtually any manmade or natural background, and which can further be designed to disguise hot regions of the target which would ordinarily be clearly discernable with infrared detection devices.
These and other objects are achieved through a system that emulates energy in the visible and infrared electromagnetic spectrum to effectively cloak an object so that the system is difficult to detect either visually or through IR detection means. More specifically, the invention provides camouflage in both the visual spectrum and the infrared spectrum by matching the infrared radiation of an object""s background and the visible radiation of the object""s background. With respect to infrared radiation matching, the invention involves sensing the temperature of an object""s background in order for the object to mimic that temperature. The external surface of the object, or alternatively a shield around the object, is heated or cooled using thermoelectric modules that convert electrical energy into a temperature gradient. When a voltage is applied to these modules one side of the module becomes hot and the other side becomes cold. By controlling the applied voltage across these modules, the temperature of the modules, and hence the temperature of an adjacent surface, can be controlled. As applied to an IR camouflage system, the output temperature of the device can be altered to match the temperature of an object""s background such that an IR viewer or detection device is unable to distinguish the object from its surrounding. Thus the object becomes thermally camouflaged.
With respect to visible radiation camouflage, the thermocouples described above are used in conjunction with choleric liquid crystals to alter the visible color of an object. Since the colors of choleric liquid crystals change with temperature, the heating and cooling effect of the thermocouples can be used to control the colors of the liquid crystals. In one embodiment, the liquid crystals are applied either directly to the surface of an object to be camouflaged or to a shield or covering surrounding the object. A color detection device such as a color detection tube is used to determine the color of the object""s background. A voltage with a certain magnitude and polarity can then be applied to the thermocouples resulting in a temperature change that alters the color of the liquid crystals to match the color of an object""s background.